curl -i -X GET "https://app.box.com/api/2.0/retention_policy_assignments/3424234/files_under_retention" \
     -H "authorization: Bearer <ACCESS_TOKEN>"

{
  "limit": 1000,
  "next_marker": "JV9IRGZmieiBasejOG9yDCRNgd2ymoZIbjsxbJMjIs3kioVii",
  "prev_marker": "JV9IRGZmieiBasejOG9yDCRNgd2ymoZIbjsxbJMjIs3kioVih",
  "entries": [
    {
      "id": "12345",
      "type": "file",
      "etag": "1",
      "sequence_id": "3",
      "name": "Contract.pdf",
      "sha1": "85136C79CBF9FE36BB9D05D0639C70C265C18D37",
      "file_version": {
        "id": "12345",
        "type": "file_version",
        "sha1": "134b65991ed521fcfe4724b7d814ab8ded5185dc"
      }
    }
  ]
}